Normally a Mobile Station (MS) in active mode in a cellular wireless communication system is handed over from one cell to the next as it moves through the system, and data can be transmitted and received without significant interruptions due to these handovers.
A handover (HO) procedure can consist of many steps. In many cellular wireless communication systems a HO is: 1) network controlled, i.e. the MS is commanded by the network when to connect to another cell; 2) prepared, i.e. the target cell to which the MS is moving to is prepared; and 3) MS assisted, i.e. the MS provides measurement reports before HO to the serving cell to assist the decision to do HO preparation of target cell(s), and when to leave the serving cell/connect to the target cell.
In the context of HO, the serving cell before HO is often referred to as the source cell. After successful HO the target cell becomes the new serving cell. In Long Term Evolution (LTE) the HO is a “hard handover”, which means that the UE radio link is switched from one (source) cell to another (target) cell. In Universal Mobile Telecommunications System (UMTS) hard HOs are used exclusively for TDD mode, but may also be used for FDD mode.
In the following discourse, the focus is on the intra frequency LTE HO procedure, but the procedures are similar for the LTE inter Radio Access Technology (RAT) and LTE inter frequency HO procedures. The intra E-UTRAN in RRC_CONNECTED state is a User Equipment (UE) assisted network controlled HO, with HO preparation signalling in E-UTRAN.
A HO is initially triggered by a measurement report sent from a UE to a serving base station, such as an eNB (E-UTRAN NodeB). The serving eNB configures how the UE shall take measurements, and under what conditions a measurement report shall be triggered and sent to the eNB.
To assist mobility control decisions, the UE can measure several different cells and report the results to the network. Different networks and network deployments can have different detailed behaviour, but in most systems it is natural to trigger HO when signal reception from a target cell is better than from a source cell.
For the case of intra-frequency HO in a reuse-one system (i.e. in a system where the source cell and the target cell uses exactly the same frequency resources) there are strong interference management benefits in (always) keeping the UE connected to the cell with the best signal strength. In the measurement report, the UE includes the reason for the trigger of a HO, e.g. target cell signal stronger than serving cell signal, and measurements of a Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ) of the serving cell and several neighbour cells including the target cell. To reduce ping-pong effects where a UE is handed over repeatedly between two cells a HO offset is often added to the HO trigger condition: target cell signal should be better than the serving cell signal by an offset, wherein the offset value >0 dB. FIG. 1 shows a typical scenario in which a UE performs mobility measurements.
When a serving eNB receives a measurement report from a UE, the eNB shall first decide whether a HO shall take place or not. This decision can be based on a single or on multiple UE measurement reports, but also on other information available in the eNB, such as information about the subscriber, UE capabilities, etc. If the eNB wishes to HO the UE to another cell, the eNB performs a HO preparation to that cell. HO preparation involves a signalling exchange between one (serving) eNB and another (target) eNB. The source cell requests the HO (Handover Request) and passes over UE context information; and the target cell decides if it can admit the UE (Call Admission Control) and either accepts or rejects the HO. In an acceptance message (Handover Request Ack), the target cell includes parameters required by the UE to allow it to communicate to the target cell—these parameters being grouped into a transparent container. The source cell may prepare multiple cells for HO.
Following a successful preparation, the HO execution takes place. The source cell issues a HO Command to the UE—this is the RRCConnectionReconfiguration message and carries the transparent container. If, and when, the UE receives this message correctly the UE synchronises to the new target cell and sends a synchronisation message on the Random Access Channel (RACH). The target cell then issues an allocation to the UE so that the UE can send a HO confirmation message to the target cell (RRCConnectionReconfiguration-Complete message).
In the final steps (Handover Completion), which do not involve the UE, the source eNB (serving the source cell) is able to forward data (un-acknowledged downlink packets) to the target eNB (serving the source cell), and the S1-U interface from the Serving Gateway (S-GW) must be switched from the source to the target cell (“path switch”). Finally, if the HO is successful, the target eNB issues a UE Context Release message to the source eNB.
The hard HO in UMTS is very similar in many respects—it is also UE assisted but network controlled (i.e. the UE is configured to send triggered measurement reports but the network decides when to execute a HO), exploits preparation (using RL Setup procedure), is a “backward” HO which means that the source cell sends the HO command to the UE and the UE replies to the target cell, and is completed by inter-node signalling.
Moreover, in 3GPP there has been considerable study into Self-Organising Networks (SON) for LTE. One part of this is the HO Parameter Optimisation also known as the above mentioned MRO which is aiming at optimising mobility parameters. The intention is to optimise the HO behaviour, for example by adjusting the measurement configuration of the UE or by adjusting the behaviour of the HO decision algorithm. It has not been specified which HO parameters shall be optimised, but examples include the HO hysteresis (also called offset) and the Time-to-Trigger (TTT) parameters. The aims of the optimisation are to reduce HO failures whilst at the same time not having more HOs than are necessary. The MRO functionality is distributed in the Evolved-UTRAN (E-UTRAN), i.e. every eNB has its own MRO optimisation function. To assist optimisation, signalling has also been defined between eNBs to help identify HO failure events.
The following is the text describing the use-case of HO Parameter Optimisation also known as MRO in section 22.5 of specification 3GPP TS36.300, 9.2.0:                One of the functions of Mobility Robustness Optimization [MRO] is to detect RLFs that occur due to Too Early or Too Late Handovers, or Handover to Wrong Cell. This detection mechanism is carried out through the following procedures:                    [Too Late HO] If the UE attempts to re-establish the radio link at eNB B after a RLF at eNB A then eNB B may report this RLF event to eNB A by means of the RLF Indication Procedure.            [Too Early HO] eNB B may send a HANDOVER REPORT message indicating a Too Early HO event to eNB A when eNB B receives an RLF Indication from eNB A and if eNB B has sent the UE Context Release message to eNB A related to the completion of an incoming HO for the same UE within the last Tstore_UE_cntxt seconds.            [HO to Wrong Cell] eNB B may send a HANDOVER REPORT message indicating a HO To Wrong Cell event to eNB A when eNB B receives an RLF Indication from eNB C, and if eNB B has sent the UE Context Release message to eNB A related to the completion of an incoming HO for the same UE within the last Tstore_UE_cntxt seconds. The indication may also be sent if eNB B and eNB C are the same and the RLF report is internal to this eNB.                        The detection of the above events is enabled by the RLF Indication and Handover Report procedures.        The RLF Indication procedure may be initiated after a UE attempts to re-establish the radio link at eNB B after a RLF at eNB A. The RLF INDICATION message sent from eNB B to eNB A shall contain the following information elements:                    Failure Cell ID: PCI of the cell in which the RLF occurred;            Reestablishment Cell ID: ECGI of the cell where RL re-establishment attempt is made;            C-RNTI: C-RNTI of the UE in the cell where RLF occurred.            shortMAC-I (optionally): the 16 least significant bits of the MAC-I calculated using the security configuration of the source cell and the re-establishment cell identity.                        eNB B may initiate RLF Indication towards multiple eNBs if they control cells which use the PCI signalled by the UE during the re-establishment procedure. The eNB A selects the UE context that matches the received Failure cell PCI and C-RNTI, and, if available, uses the shortMAC-I to confirm this identification, by calculating the shortMAC-I and comparing it to the received IE.        The Handover Report procedure is used in the case of recently completed handovers, when an RLF occurs in the target cell (in eNB B) shortly after it sent the UE Context Release message to the source eNB A. The HANDOVER REPORT message contains the following information:                    Type of detected handover problem (Too Early HO, HO to Wrong Cell)            ECGI of source and target cells in the handover            ECGI of the re-establishment cell (in the case of HO to Wrong Cell)            Handover cause (signalled by the source during handover preparation)                        
MRO is expected to meet a specified HO failure rate target, and to minimize the number of HO events whilst meeting this failure rate target. In this respect, the need to control MRO behaviour is important. By controlling the MRO, the system operator can control the behaviour depending on deployment phase and possibly also depending on the performance and/or stability of the MRO algorithms for different eNBs. The relationship between OAM, MRO HO algorithm, and the UE is depicted in FIG. 2. In this figure, it is shown that the OAM controls the MRO, which in turn controls the HO algorithm thereby controlling the UE behaviour by controlling the measurement configuration for the UE.
It has been suggested to let the OAM directly control the automatic optimization of mobility parameters in the eNB by specifying restrictions on the output parameters of the MRO function (measurement configuration). The following parameters have been suggested to be controlled: Hysteresis, TTT and Cell Individual Offset (CIO). This can be summarized as a solution where the OAM selects a set of valid parameters and the eNB (or MRO) selects one of these parameters.
However, problems with prior art are that different eNBs may choose to have different behaviour of the HO algorithm, using different measurement configurations and different input used for the HO decision. If an operator uses OAM to control the output of the HO algorithm, it assumes that the operator has a detailed knowledge about the algorithm using the output parameters, i.e. the HO algorithm. By restricting the range of the measurement configuration parameters there is a risk that it may limit the performance of the proprietary HO and MRO algorithms.
In order to avoid such a situation, the network entity modifying the parameters needs to have a detailed knowledge about the actual implementation of the HO algorithm. This further requires that in a network comprising network equipment from multiple providers, there is a need to set different working points and ranges for equipment from different vendors. Also, different vendors may let the MRO and HO algorithms modify different HO parameters so the network operator also needs to understand in detail which HO parameters should have their range controlled. For example, vendor A may let the MRO and HO algorithm only adjust the CIO parameter, whilst vendor B may modify the TTT parameter. Vendor C may modify both CIO and TTT but certain pairs of values within the total set of paired values formed from the specified permitted ranges for CIO and for TTT may be excluded by the proprietary MRO or HO algorithm in the eNB since these pairs of values are considered as not beneficial. For example, CIO may take values −3 and −4, and TTT 0 and 160 ms, but {CIO, TTT} values {−3, 0}, {−3, 160} and {−4, 0} would be valid but not {−4, 160}. If OAM should control the output of the HO algorithm, this would more or less create a need for transparency between OAM all the way to the measurement configuration.
There is therefore a need for an improved method in the art which mitigates and/or solves the disadvantages with prior art solutions.